Soups-n-Salads is a sizeable buffet restaurant chain with 57 locations in the Pacific Northwest, offering its customers a choice of 5 different soups and 12 gourmet salads. As part of

a continuous effort to present its customers with food items that are fresh, delicious, and safe, Soups-n-Salads follows rigorous quality control procedures developed by their inhouse Six Sigma team. Part of this effort addresses the problem of temperature management for food items. Danger Zone. Bacterial growth on food is a serious public health hazard. The U.S. Department of Agriculture estimates that as many as 9,000 deaths and 6.5 to 33 million illnesses yearly are directly linked to foodborne bacteria and other microorganisms. To survive and reproduce, bacteria need time, moisture, and a favorable temperature. One of the most critical factors for the successful prevention of bacterial growth is storing food at the right temperature. Long term practice and lab experiments have shown that bacteria grow and multiply faster in temperatures in the 40 – 140 °F range (5 – 60 °C). This range is known as the Danger Zone. Safe food storage practices and regulations require from grocery stores and restaurants to take all the necessary steps to avoid a prolonged exposure of food in the danger zone. Generally, cold temperatures (below 40 °F) will significantly slow down bacterial growth, and hot temperatures (above 140 °F) will destroy most of the bacteria. Bacteria do not affect the taste, smell, or appearance of a food. Therefore, if a food has been left in the danger zone for more than 2 hours should be discarded, even if it smells or looks good. Soups and Salads. Safe storage and display of salads at a buffet restaurant involves keeping the salads in containers that will protect them from possible exposure to insects or rodents, avoiding cross-contamination from other food, proper refrigeration, isolation from cleaning agents, practicing stock rotation (i.e., consuming the older stock first), and other practices. Temperature management for salads is relatively simple, due to the fact that most salad items remain at a low temperature until they are consumed or disposed of. Keeping soups at a safe temperature can be more challenging, since they need to be heated when they are cooked, chilled when they are stored, and reheated when they are about to be consumed by the customers. Soups pass through the danger zone twice, and this transition needs to be as fast as possible, to prevent bacteria from having the time or the opportunity to grow. The Process Flow Diagram shown in Exhibit 1 illustrates the temperature management procedures for soups at Soups-n-Salads restaurant. Statistical Quality Control. Soup temperature at each Soups-n-Salads restaurant is monitored every half hour during the period 11:30am - 10pm, for a total of 22 measurements per day. Two restaurant employees per day share two daily shifts recording the temperatures of 12 different soup containers (bain-maries). A temperature log of the most recent 30 days is maintained, as required by the Health Department. An effort is made to keep the soup temperature above 150 °F, just to be on the safe side. Out of the 12 soups measured every half-hour, four are randomly chosen to be included in a control chart showing the average soup temperature of the sampled bain-maries. Exhibit 2 shows the first few observations of sampled temperatures corresponding to two consecutive days. Occasionally all four soups in the sample have temperatures that are unusually low and some other times just one or two temperatures are low and the rest are OK. Depending on whether a sample has similar low temperatures or just large differences among the temperatures it consists of, the managers of Soups-n-Salads may be able to get a head start in trying to figure out a possible cause. For example, if the temperature in all soups is dropping it might be an indication that the employee responsible for stirring the soups was not making the rounds, whereas if only one has a low temperature it might relate to a machinery malfunction. Exhibit 3 shows a cause-and-effect (fishbone) diagram, created in order to transcribe a number of possible causes for a low soup temperature, as they have been known to the managers through long experience. Boiling hot hot cold Cook soup. Temperature at boiling is slightly above 212 °F Divide soup into shallow containers for rapid cooling. Place soup in plastic bags and Refrigerate within 2 hours Danger Zone, Temperature 45 Heat soup fast. Raise temperature to 160 °F 4 EXHIBIT 1: Process Flow Diagram showing the temperature management procedures for soups at Soups-n-Salads Restaurant. 5 Preheat the bain- marie before use Keep the soup hot in a bain-marie. Operate at the highest setting. Stir soup frequently. Take temperature every half hour. Maintain above 140 °F. Keep soup in the bain-marie for a max of 1 hour, then dispose Store in refrigerator and use within 4 days. Keep temperature below 40 °F Sample temp1 temp2 temp3 temp4 1 149.8 148.9 150.2 149.3 2 154.1 150.4 153.2 155.8 3 151.2 155.4 152.2 153.9 154.1 151.9 153.9 153.3 153.1 157 156.7 153.2 EXHIBIT 2: Sampled temperatures for two consecutive days (first few observations) People Employees don't care Soup is not stirred often Power outage Environment Equipment Thermometer not measuring properly Instruments are old and need replacement Original heating not high enough Bain-marie preheating not high enough Procedures EXHIBIT 3: Cause-and-effect (fishbone) diagram. Bain-marie not working properly Poor maintenance Soup temperature not high enough Refrigerator temperature too low Using statistical software, the Six Sigma team produced an R chart and an x-bar chart for the soup temperature. Examination of the R chart showed that the process was out of control with respect to the variability on the 27th sample. At that time, one of the four sampled soups was getting much colder than the other three. Later, the Six Sigma team produced a new R chart, along with a corresponding x-bar chart, both shown in Exhibit 4. Range X-bar 2 10 8 O 157 155 153 151 149 2 3 4 5 6 7 8 9 10 MA 0 Range Chart for temp 0 10 10 d₂ 1.128 1.693 2.059 2.326 2.534 2.704 2.847 20 2.970 3.078 x Subgroup 20 X-bar Chart for temp 30 d3 .853 Subgroup .888 .880 .864 .848 .833 .820 .808 .797 As 30 EXHIBIT 4: R chart and X-bar chart (second round of control charts). 40 AMA D3 40 0 0 0 0 .076 .136 50 .184 .223 UCL = CTR= 4.12 LCL = 50 D₂ 3.267 2.574 2.282 2.114 2.004 1.924 1.864 1.816 1.777 UCL = CTR= 153.78 LCL =